The present invention relates to a method for oil treating insoluble sulfur. More particularly, the present invention relates to a method for oil treating insoluble sulfur wherein the stability of the insoluble sulfur is not adversely affected by the oil treatment. Specifically, the present invention relates to a method of inhibiting the destabilizing effect of an oil on insoluble sulfur which is treated with the oil.
Insoluble sulfur is, by definition, sulfur which is insoluble in carbon disulfide. This form of sulfur is generally understood to be polymeric in nature, the polymer chains being made up of up to several thousand sulfur atoms. Insoluble sulfur is distinguished from soluble sulfur, which is crystalline in form.
As a practical matter, most commercial grades of insoluble sulfur contain both soluble and insoluble sulfur. Commercial insoluble sulfur products with varying amounts of insoluble sulfur are available.
The most important use for insoluble sulfur is as a vulcanizing agent in the rubber-making industry. In this industry, sulfur is used as a cross-linking (vulcanizing) agent in rubber compound formulations.
Although soluble sulfur would fulfill the basic crosslinking requirements for the rubber formulations, the inclusion of soluble sulfur in rubber compositions results in processing difficulties which, in turn, cause defects in products made from such compositions. The two most common problems caused by the use of soluble sulfur are "bloom" and "bin scorch".
Sulfur bloom is the crystallization of sulfur on the surface of rubber articles. This is caused by the migration of sulfur from the interior of a rubber article to the surface. This phenomenom results because the sulfur is soluble in the rubber at mixing (masticating) temperatures, but when the mixture is cooled after mixing, the solubility limits are reduced and a supersaturated solid solution is formed. As the mixture cools and the lower solubility limits become controlling, sulfur beings to come out of the solid solution. As it comes out of solution, the sulfur migrates to the surface of the rubber article and crystallizes.
This surface bloom becomes a serious problem because it destroys the natural tack of the rubber in the affected area. When several plies of rubber are assembled together to construct tires, belts, hoses or the like; the sulfur bloom interferes with the natural cohesion between adjacent plies and faults, such as air bubbles, result.
When insoluble sulfur is used, however, surface bloom is eliminated. Insoluble sulfur becomes evenly distributed throughout the rubber composition during the mixing (mastication) step, but does not go into solution in the rubber composition. The distribution of the insoluble sulfur remains as it was when dispersed, there is no concentration gradient formed; and migration does not occur.
It is only when the final article is vulcanized that the insoluble sulfur reverts to a soluble form and goes into solution. The vulcanization reaction takes place at the same time and the sulfur becomes part of a high-polymer product. The sulfur thus becomes chemically bound and cannot bloom upon cooling of the rubber mass.
Another serious problem caused by the use of soluble sulfur in rubber formulations is that of bin scorch. Bin scorch occurs when an uncured rubber composition is kept in storage after having been fully compounded (i.e., mixed, milled, etc.). The uncured rubber composition is temperature sensitive, and curing (i.e., crosslinking) may be prematurely initiated to some extent if storage temperature is not properly controlled. This is known as "bin scorch".
When insoluble sulfur is used, bin scorch is retarded to a point where it ceases to be a problem. Since insoluble sulfur is available for reaction only when curing (vulcanizing) temperatures are reached, no significant reaction takes place at lower temperatures. At these lower temperatures the insoluble sulfur merely remains as a suspended solid surrounded by rubber, until vulcanization takes place.
Because of the technical advantages offered by insoluble sulfur as compared to soluble sulfur, a great deal of technical activity has been devoted to the development of methods for producing insoluble sulfur in preference to soluble sulfur. This effort has been successful and at the present time there are a number of methods known in the art by which sulfur products comprising 90% insoluble sulfur or more can be produced.
Unfortunately, however, the insoluble form of sulfur is metastable and tends to revert, over a period of time, to the soluble form. Typically, an unstabilized sulfur product having about 90% insoluble sulfur will revert to a product having only about 70 to about 75% insoluble sulfur over a period of about 20 hours when stored at a temperature of about 60.degree. C.
A number of stabilizers have been developed to retard the reversion of insoluble sulfur to the soluble form. These stabilizers, in general, are capable of reducing the reversion rate by about 80%.
Insoluble sulfur, in addition to being treated with a stabilizer to retard reversion, is also usually treated with an oil to control the tendency of the sulfur (often in fine particulate form) to form dust and also to make it easier to disperse in organic compositions with which it is to be blended. Oil-treated insoluble sulfur products can contain from about 1% up to about 30% oil (by weight) and from about 99% to about 70% sulfur (by weight). At these ratios, the product remains dry to the touch and can be handled as a dry powder.
Unfortunately, however, some oils tend to increase the reversion rate of the insoluble sulfur even though it has been "stabilized". The mechanism by which oil-treatment of stabilized insoluble sulfur causes it's reversion rate to increase is not fully understood. Moreover, it is difficult to predict whether any given processing oil will or will not affect the stabilized insoluble sulfur in this way. In actual practice it has become customary to screen each batch of oil by actually treating a small quantity of stabilized insoluble sulfur with it and measuring its effect on the reversion rate.
In addition to the problem of screening oil batches for use in oil-treating sulfur, producers are faced with the possibility that oil which does not adversely affect stability may not always be readily available. As the general availability of petroleum products continues to diminish, these problems may be expected to become more severe.
It is therefore highly desirable that a method be found for inhibiting the tendency of processing oils to accelerate the reversion rate of insoluble sulfur. With such a method available, a much greater variety of oils could be used for oil-treating sulfur, and the ultimate sources of supply for such oils would be widely expanded.